The present invention relates to variable displacement hydrostatic transmissions and, more particularly, to a pressure relief valve providing a wide neutral feature in a hydrostatic transmission.
Generally, hydrostatic transmissions consist of a variable displacement hydraulic pump unit driven by an internal combustion engine of the vehicle, and a fixed displacement hydraulic motor unit connected to the drive train of the vehicle. A closed fluid loop consisting of first and second conduits fluidly connect the pump with the motor. The motor speed is zero when the fluid displacement of the pump is zero, which corresponds to a neutral position. By increasing the fluid displacement of the pump, the pump drives the motor in a direction consistent with the fluid flow direction within the first and second conduits. The fluid displacement of the pump can be in either direction which changes the flow of fluid in the first and second conduits by creating a difference in pressure between the two conduits. Thus, the motor is caused to rotate in a first and second direction corresponding to forward and reverse directions.
Further, the displacement of fluid within the pump is variably controlled by the operator to whereby change the volume of fluid flow within the closed conduit loop between the pump and the motor. Thus, the pump and therefore the output of the motor can variably range from a full reverse position, through a neutral position, to a full forward position.
Such a hydrostatic transmission of the type described above and contemplated for use with the present invention is shown in U.S. Pat. No. 5,078,659 issued to Von Kaler et al., and incorporated herein by reference.
Variable displacement hydrostatic transmissions have come to be utilized in a variety of applications, most notably in lawn and garden tractors. A hydrostatic transmission provides a variable speed drive which is continuously shiftable from a forward to a reverse position, with neutral therebetween. Because the closed conduit loop is pressurized, changes in the displacement of the pump causes abrupt pressure changes in the loop as the fluid displacement of the pump is moved from a neutral position into a forward or reverse position, or from a forward or reverse position into a neutral position. The abrupt pressure change is caused by the reversal of fluid power flow within the conduits during shifting from a forward to a reverse position or vice versa. Also, abrupt pressure changes can occur during initiation of fluid flow within the conduit upon displacement of the pump from the neutral position. The abrupt pressure changes are further magnified by the rate at which the pressure changes are made which correspond to the rate at which the operator moves the pump displacement control.
In addition, the effects of abrupt pressure changes within the conduit loop are more pronounced at low pump displacements versus at high pump displacements. Thus, the pressure changes are more acute when the displacement of the pump is near zero, or near the neutral position.
Such abrupt pressure changes are not desirable in a transmission since they cause lurching of the vehicle as the transmission is engaged from neutral into forward or reverse. Also, abrupt pressure changes cause fatigue of the transmission components as surges of fluid are displaced within the conduits, pump, and motor.
Further, when the transmission is in a neutral position, there may be the tendency of the transmission to "creep" such that the transmission will tend to slightly move in either the forward or reverse direction. This occurs when displacement of the pump is not completely zero, such as can happen if the pump displacement control mechanism is not completely synchronized with the operator control, as well as other causes.
It is thus desirable, both from the standpoint of operator comfort and transmission life, to reduce or eliminate the pressure surges which naturally occur during operation of the hydrostatic transmission. It is also desirable to alleviate creeping by providing a wider and more positive neutral position.
U.S. Pat. No. 3,451,218 issued to Grant utilizes a compensator valve disposed in a conduit connecting the two sides of a hydrostatic loop in order to provide a short circuit between the high and low pressure conduits. The valve is keyed via linkage to the displacement of the pump such that activation of the valve is accomplished with the displacement of the pump.
The prior art has not provided a simple, effective pressure reduction mechanism which does not sacrifice transmission power efficiency.
It is therefore an object of the present invention to reduce the abrupt pressure changes within the conduit loop as the pump displacement is moved into and out of a neutral position.
It is another object of the present invention to provide a wide neutral feature in a hydrostatic transmission within a predetermined range of movement about a neutral pump position.
It is yet another object of the present invention to provide pressure reduction in a hydrostatic transmission and a wide neutral which is coordinated with the movement of the pump displacement control mechanism.
It is still another object of the present invention to provide a wide neutral feature in a hydrostatic transmission which does not sacrifice the full power efficiency performance of the transmission.